Thermal history gage



May 11, 1965 N. E. ROGEN THERMAL HI STORY GAGE Filed Nov. 50. 1960 FIG!RESISTIVITY l IOO hrs 1+- 500 hrs I l l \wxua'mm INVENTOR. NEIL E. ROGENATTO R N EYS United States Patent 3,182,507 THERMAL HISTORY GAGE Neil E.Regen, Cambridge, Mass, assignor to Iiilron, Corporation, Natick, Mass,a corporation of Delaware Filed Nov. 30, 1960, Ser. No. 72,661 14Claims. (Cl. 73-362) This invention relates to temperature measuringdevices, and more particularly, to a novel and improved thermal historygage for determining the time interval of operation of a device at orabove a predetermined temperature.

It is often desirable or necessary to determine how long a device hasbeen operated at or above a predetermined temperature. For example, inan aircraft jet engine, prolonged overheating of the turbine blades maycause a transformation of the turbine blade material with an attendantloss in the high temperature-strength characteristic of the material.Accordingly, military and commercial service requirement for jet engineservice inspection provides that the engine must be disassembledperiodically, and portions thereof X-rayed. Also, it is a commonpractice to require sectioning of one of the turbine blades formetallurgical examination. Such a procedure is, of course, costly andresults in the aircraft being removed from the operational category.While it might be possible to utilize known temperature measuringdevices and recording instruments to determine the time interval ofoperation of the engine about a critical temperature, suchinstrumentation would be costly, and would undesirably decrease the payload of the aircraft.

Accordingly, it is a primary object of this invention to provide a noveland improved means for determining the thermal history of an environmentwhich does not require the use of complex and costly temperaturemeasuring and recording instrumentation.

It is a further object of this invention to provide a novel and improvedmeans for determining the thermal history of an environment which issuitable for use in locations which would normally be inaccessible withregard to conventional temperature measuring and recording equipment.

Other objects will be in part obvious, and in part pointed out more indetail hereinafter.

The objects of the present invention are achieved in a novel andimproved thermal history gage characterized by an electricallyconductive sensing element which is fabricated from a material whichbelow a predetermined threshold temperature is stable and which at orabove the threshold temperature undergoes a transformation as a functionof temperature and time. This transformation results in a change inphysical properties of the material including a change in resistivity ofthe material. By proper selection of the alloy and the heat treatmentthereof, the resistance of the sensing element will provide anindication whether or not the gage has been exposed to a temperatureequal to or exceeding the threshold temperature. In use, the gage ismerely located within the environment, the thermal history of which itis desired to determine. After a selected interval of time, theresistance of the sensing element is measured either with the gage inplace or after removal of the gage from the environment. From theresistance of the sensing element, it may then be determined whether thetemperature of the environment in which the gage was installed exceededa preselected temperature, and if so, for how long. A more completeunderstanding of the invention may be had with reference to thefollowing detailed description when taken in connection with thefollowing detailed drawing:

FIG. 1 is a graph illustrating the relationship of electricalresistivity of an age hardenable nickel-chrome alloy to the time ofaging of the material at a particular temperature;

FIG. 2 is a graph illustrating the relationship of the electricalresistance of a sensing element of a thermal history gage of thisinvention with respect to time and temperature;

FIG. 3 is a cross sectional view of a thermal history gage constructedin accordance with the present invention; and

FIG. 4 is a cross sectional view substantially along the line 44 of FIG.3.

As heretofiore indicated, a thermal history gage of the presentinvention is characterized by a temperature sensing element fabricatedfrom material which has the characteristic that, after suitable heattreatment, exposure of the sensing element to a temperature equal to orexceedmg a certain threshold temperature will result in a predictablechange in resistance of the sensing element as a function of time andtemperature. A group of materials which have this desired characteristicare the so called age or precipitation hardenable alloys. The terms ageor precipitation hardenable alloys is used herein to define alloys whichconsist essentially of a base material and one or more alloying elementswhich will, after suitable heat treatment, provide a secondary phase inprecipitate particle form with the precipitate consisting essentially ofa compound of the base metal and one or more of the alloying elements.Such alloys have found particular use in providing structures which havehigh strength at elevated temperatures. The increased strength andhardness of these alloys results from the precipitation of compounds ofthe base metal, and at least one of the alloying elements when the alloyis aged at elevated temperatures. Accordingly, such alloys are generallyreferred to as being age hardenable, or precipitation hardenable alloys.A well known group of age hardenable alloys are those comprising metalssuch as nickel and chromium with additional minor percentages ofhardening constituents such as titanium and aluminum as well as otherelements such as carbon, columbian, molybdenum, tungsten, etc. it beingunderstood that all of the additional elements do not necessarily occurin every alloy. It will, of course, be understood that there are manyother alloys other than those set forth above which fall within thecategory of age or precipitation hardening alloys.

The controlled heat treatment of an age or precipitation hardenablealloy is normally accomplished after the alloy has been manufacturedinto the final product. The heat treating of articles fabricated of agehardenable alloys normally comprises the solution treatment of the partfollowed by a second heat treatment to accomplish the aging desired. Anintermediate heat treatment may or may not be performed depending on theend use of the article. The solution treatment of the article involvesthe heating of the alloy to a relatively high temperature under themelting point of the alloy, for example, 2000 F. until all of theconstituents of the alloy are in solid solution. The alloy is held atthe solution temperature for a rela tively short time, for example,approximately two hours. The alloy is then cooled, usually in air,although an oil or water quench may be utilized in some instances. Thealloy is subsequently aged by heat treatment, say at 1500 F., for aperiod of up to 24 hours. Such aging causes a precipitation of the basemetal and an alloying element in a complex compound and imparts a highstrength to the alloy at elevated temperatures. This precipitation ofthe base metal-alloying element compound is dependent upon temperatureand time. In subsequent use of the alloy as long as the temperature ofthe alloy is maintained below the temperature at which it was aged, thestructure of the alloy will remain substantially the same. However, ifthe alloy is raised to or above the aging temperature and held there, atransformation in the form of additional precipitation will occur, andthe physical properties of the alloy Will change.

One of the characteristics of an age hardenable alloy is that itselectrical resistivity varies generally in the same manner as thehardness of the alloy. Thus, for a particular alloy, the resistivitywill vary with time during the aging heat treatment of the alloy. Thegraph of FIG. 1 illustrates this relationship of electrical resistivityto aging time in a nickel-chrome-aluminum age hardenable alloy. It canbe seen that at the aging temperature the resistivity of the alloy willincrease for a certain initial period of time until it reaches amaximum, and then will decrease over a period of time which issubstantially larger than the initial period during which theresistivity was increasing. As can be seen from FIG. 1, the aging timerequired for the resistivity of the alloy to reach its peak value andbegin to decline may be somewhat in excess of 100 hours. Resistivity ofthe alloy will then further decline in an asymptotic manner for a periodwhich may exceed 500 hours. It will be observed that after theresistivity begins to decline, the curve will be substantially linearfor a prolonged period.

If the alloy is quenched at any point on the curve of FIG. 1, theproperties of the alloy will be retained substantially the same as thatwhich existed immediately prior to quenching. Subsequently, as long asthe temperature of the alloy is below the aging temperature, the alloywill be relatively stable, in that substantially no furtherprecipitation will occur. However, if the temperature of the alloy israised to or above the threshold temperature, additional precipitationwill occur and the resistivity of the alloy will again begin to varyalong the curve of FIG. 1 as a function of temperature and time and willcontinue to do so as long as the temperature remains equal to or abovethe threshold temperature. It will be apparent from the curve of FIG. 1that if the alloy is quenched after the resistivity peak has beenreached, the alloy may be made to have a resistivity variation which issubstantially linear over a prolonged period when and if the alloy isagain heated to or above the threshold temperature.

In a preferred embodiment of this invention, a sensing element isfabricated from a thin wire of nickel-chrome age hardenable alloy. Thesensing element is then solution treated and finally aged in the mannerlast described to provide a relationship of resistance and service timeas shown in FIG. 2. With reference to FIG. 2, the initial resistance ofthe gage is set by the point on the curve of FIG. 1 at which quenchingoccurred. This initial resistance will remain substantially constant attemperatures below the threshold temperature as shown by the 1200 F.line of FIG. 2. However, the sensing element resistance will vary in asubstantially linear manner. when the gage is heated to or above thethreshold temperature, which as shown in FIG. 2 is 1300" F. It will benoted that the variation of sensing element resistance will becumulative in the case of an environment which cyclically varies aboveand below the threshold temperature. Thus, the resistance of the sensingelement will provide an indication of the thermal history of theenvironment in which the sensing element was installed. 7

With reference to FIGS. 3 and 4, a specific embodiment of a thermalhistory gage of the present invention comprises a sealed housing 10including a base 12 and an enclosing body 14. The housing is fabricatedof a suitable glass or other high temperature insulation material whichis electrically non-conductive. Mounted in the housing and on the base12 are a pair of support members 16 also fabricated of material such asthat of the housing. In the gage of FIGS. 3 and 4, a plurality ofelectrically conductive sensing elements 18, 20, and 22 are suitablymounted on the support 16. A plurality of pairs of terminals 24 extendthrough opposite sides of the housing and are connected respectively tothe ends of the sensing elements. The terminals extend outwardly throughseals in the housing to permit measurement of the resistance of thesensing elements from externally of the housing. The materials fromwhich the housing and supports are fabricated should preferably be suchthat they will not react with the material of the sensing elements atelevated temperatures so as to preclude any undesired variation of thealloy of the sensing elements. Further, the housing is preferably filledwith argon or any other suitable inert atmosphere.

Returning to the sensing elements, these components are preferably ofelongated thin configuration, and in the specific embodiment, are wiresof approximately fifteen mils. The sensitivity of the gage will bedetermined, to a large extent, by the size of the wires, and the smallerthe wire, the greater the sensitivity. The alloys utilized in thesensing element of FIGS. 3 and 4 have an initial resistivity ofapproximately 750 ohms per mil foot and an initial resistance ofapproximately 10 ohms. The alloys of the sensing elements 18, 20, and 22are of different composition in order to provide diiierent thresholdtemperatures for the sensing elements, thus providing a gage which willindicate the thermal history of an environment with respect to aplurality of temperatures. For example, the sensing element 13 may befabricated from an age hardenable alloy consisting essentially of 2%aluminum with a balance of nickel and chromium in an 20 relationship.The percentages given are atomic percentages. There may, of course, beminor amounts of additional elements present in the alloy. Such analloy, after solution treatment, may be aged at 1300 F. in order toobtain the desired variation in resistivity both during and after theaging treatment.

The normal aging treatment might be, as indicated above, for a period upto 24 hours. However, it is preferred that in aging a sensing element ofthis invention, the aging treatment proceed for a time sufiicient forthe resistivity of the material to reach its peak and decline into thesubstantially linear portion of the curve of FIG. 1. This may requireaging up to hours. Accordingly, the material may be said to be overaged.It should be understood that the term overaging, as used herein,actually refers to an aging period substantially larger than thatnormally used and is not used to refer to an intermediate treatmentbetwen solution treating and aging. The sensing element is preferablyfabricated prior to the aging treatment, and it is, of course,understood that the sensing element is solution treated prior to aging.The time between solution treatment and aging is not critical and may beseveral days or longer.

In order to provide sensing elements for various threshold temperatures,the solute content is varied. More speciiically, the lower the solutecontent the lower the temperature at which precipitation Will begin. Thesensing element 20 of FIGS. 3 and 4 is fabricated from an alloyconsisting essentially of 2% aluminum, 2% titanium and the balance is80-20 nickel and chromium. This alloy may be aged at 1400 F., thusproviding a sensing element having a threshold temperature of 1400 F.The sensing element 22 has a threshold temperature of 1500 F. and isfabricated from an alloy consisting essentially of 3% aluminum, 3%titanium and the balance 80-20 nickel and chromium. The sensing elements2%) and 22, with the exception of the aging temperature, are heattreated in the same manner as the sensing element 18; that is to say,they are solution treated and then overaged for a period up to 100 hoursor until the resistance of the sensing element has reached the desiredvalue on the right hand side of a curve such as that of FIG. 1.

In service, the gage is installed in a location, the thermal history ofwhich it is desired to determine. After a suit able service period, theresistances of the various sensing elements are measured. Let us assumethat the resistances of the elements 18 and 20 have changed while theresistance of sensing element 22 has remained the same. This will thusindicate that the temperature of the environment did not reach 1500 F.The resistance change of the gage will provide an indication of the timeof operation at or above 1400 F., or in other words, between 1400 F. and1500 F. The resistance of the gage 18 Will, on the other hand, providean indication of the time of operation above l300 F. including the timeof operation between 1400 F. and 1508" F. By means of prior calibrationof sensing elements of the particular type concerned, we may determinethe efiect on the element 18 of the time of operation above 1490" F. asindicated by the element 2%. This may be applied to the time-temperatureindication provided by the element 18 so as to provide an indication ofthe time of operation between 1300 F. and 1400 F. 7

Thus, it can be seen that there has been provided a novel and improvedthermal history gage for determining the thermal history of anenvironment which does not require the use of complex recordinginstruments. The gage is suitable for location in normally inaccessiblelocations inasmuch as it can be fabricated in very small sizes. Forexample, the gage of FIGS. 3 and 4 may be made to have an overall sizeapproximating that of a dime. While threshold temperatures have beenindicated as specific temperatures, it will be understood that therewill be some variations due to minor variations in alloy composition aswell as other tolerances. Accordingly, for example, Where a gage isstated as being a 1300 F. gage, the threshold temperature may actuallyvary from say 1290 F. to 1310 F. However, it will be apparent that thisvariation will not detract from the utility of the gage inasmuch as theexact temperature is not usually particularly important. Also, asindicated above, alloys other than nickel-chromium age hardenable alloysmay be utilized in the fabrication of sensing elements. For example,included within the scope of this invention is the use of a Martensitictransformation to indicate that a temperature of an environment hasexceeded a certain value. While the Martensitic transformation isnormally quite rapid, for example, on the order of one microsecond, thesensing element material would still fall Within the requirement ofundergoing a transformation as a function of time and temperature eventhough the time Was exceedingly short. Thus, it should be emphasizedthat various substitutions and modifications in the alloys of a sensingelement of this invention, as well as various modifications and changesin the structure heretofore described, may be made Without departingfrom the scope of this invention.

Inasmuch as many changes could be made in the above construction andmany apparently Widely different embodiments of this invention could bemade without departing from the scope thereof, it is intended that allmatter contained in the above description or shown in the accompanyingdrawing shall be interpreted as illustrative and not in a limitingsense.

It is also to be understood that the language in the following claims isintended to cover all of the generic and specific features of theinvention herein described and all statements of the scope of theinvention which, as a matter of language, might be said to falltherebetween,

I claim:

1. A thermal history gage for providing an indication of the length oftime an environment has been at or above each of a plurality oftemperatures, comprising a frame,

' and a plurality of electrically conductive sensing elements carried bythe frame, each of said sensing elements being fabricated from amaterial which below a threshold temperature is stable and which at orabove a threshold temperature undergoes a transformation as a functionof temperature and time resulting in a change in physical properties ofthe material including a change in resistivity 6 thereof, each of saidsensing elements having a different threshold temperature.

2. A thermal history gage for providing an indication of the length oftime an environment has been at or above each of a plurality oftemperatures, comprising a housing, and a plurality of electricallyconductive wire-like sensing elements mounted within the housing, eachof said sensing elements being fabricated from a material which below athreshold temperature is stable and which at or above a thresholdtemperature undergoes a transformation as a function of temperature andtime resulting in a change in physical properties of the materialincluding a change in resistivity thereof, each of said sensing elementshaving a diiferenit threshold temperature, and a plurality of pairs ofterminal means carried by said housing and respectively associated withsaid sensing elements whereby the resistance of each of said sensingelements may be measured from externally of said housing.

3. A thermal history gage comprising a housing, and a plurality ofelectrically conductive sensing elements sealed Within the housing andeach fabricated from an averaged age hardenable alloy, each of saidsensing elements having been aged at different temperatures whereby theresistance of said sensing elements will provide an indication of theextent of time the gage has been exposed to each of .a plurality ofthreshold temperatures respectively corresponding to the agingtemperatures of the sensing elements.

4. A thermal history gage comprising a housing, and a plurality ofelectrically conductive wire-like sensing elements sealed within thehousing and each fabricated from an overaged age hardenable alloy, theaging temperatures of said sensing elements being different whereby theresistance of said sensing elements will provide an indication of theextent of time the gage has been exposed to each o a plurality ofthreshold temperatures respectively corresponding to the agingtemperatures of the sensing elements, an inert atmosphere filling saidhousing, and a plurality of pairs of terminals extending outwardlythrough the housing and respectively connected to said sensing elements,whereby the resistance of each of said sensing elements may be measuredfrom externally of said housing.

5. The method of manufacturing, for use in a thermal history gage, asensing element which will provide an indication of the exposure of thegage to a temperature equal to or exceeding a threshold temperature,comprising the steps of: fabricating a sensing element from an agehardenable alloy; heat treating the sensing element at a temperaturebelow its melting point for a time sufirc-ient to provide a solidsolution; cooling said sensing element; subsequently overaging saidsensing element at said threshold temperature for a predetermined timesuihcient for the resistance of the sensing element to reach a peak anddecline; and quenching the sensing element at the expiration of saidpredetermined time of aging.

6. A thermal history gage characterized by an electri cally conductivesensing element which below a predetermined threshold temperature isstable and which at or above a predetermined threshold temperatureundergoes a transformation as a function of temperature and timeresulting in a change in physical properties of the material of thesensing element including a change in resistivity thereof, the sensingelement prepared by fabricating a sensing element from an alloy havingthe characteristic of under-going a transformation resulting in a changein physical properties, including resistivity thereof, when the alloy isheated to said preselected threshold temperature; heat-treating saidsensing element at said threshold temperature for a predetermined timesufiicient to establish a preselected resistance of the sensing element;and quenching said sensing element upon expiration of said predeterminedtime of heat treating, whereby the resistance of said sensing elementindicates whether or not the gage has been exposed to a temperatureequal to or exceeding said threshold temperature.

7. A thermal history gage comprising a housing,- a sensing elementmounted internally of the housing and being fabricated from a materialwhich below a threshold temperature is stable and which at or above athreshold temperature undergoes a transformation as a function oftemperature and time resulting in a change in physical properties of thematerial including a change in resistivity thereof, the sensing elementprepared by fabricating a sensing element from an age hardenable alloy,heat treating the sensing element at a temperature below its meltingpoint for a time suficient to provide a solid solution; cooling saidsensing element; subsequently aging said sensing element at saidthreshold temperature for a predetermined time sufficient to establish,by precipitation, a preselected resistance of the sensing element; andquenching the sensing element upon expiration of said predetermined timeof aging.

8. A thermal history gage comprising a sealed housing containing aninert atmosphere, an electrically conductive sensing element fabricatedfrom a material which below a threshold temperature is stable and whichat or above a threshold temperature undergoes a transformation as afunction of temperature and time resulting in a change in physicalproperties of the material including a change in resistivity, thesensing elementprepared by fabricating a sensing element from an agehardenable alloy; heat treating the sensing element at a temperaturebelow its melting point for a time suficient to provide a solidsolution; cooling said sensing element; subsequently overaging saidsensing element at said threshold temperature for a predetermined timesufficient for the resistance of the sensing element to reach a peak anddecline; and quenching the sensing element at the expiration of saidpredetermined time of aging.

9. The method of fabricating, for use in a thermal history gage, asensing element which will provide an indication of the exposure of thegage to a temperature equal to or exceeding a preselected thresholdtemperature, comprising the steps of: fabricating a sensing element froman alloy having the characteristic of undergoing a transformationresulting in a change in physical properties, including resistivitythereof, when the alloy is heated to said preselected thresholdtemperature; heat treating said sensing element at said thresholdtemperature for a predetermined time sufiicient to establish apreselected resistance of the sensing element; and quenching saidsensing element upon expiration of said predetermined time of heattreating.

10. The method of manufacturing, for use in a thermal history gage, asensing element which will provide an indication of the exposure of thegage to a temperature equal to or exceeding a threshold temperature:comprising the steps of fabricating a sensing element from an agehardenable alloy, heat treating the sensing element at a temperaturebelow its melting point for a time sufficient to provide a solidsolution; cooling said sensing element; subsequently aging said sensingelement at said threshold temperature for a predetermined timesuflicient to establish, by precipitation, a preselected resistance ofthe sensing element; and quenching the sensing element upon expirationof said predetermined time of aging.

11. A method of determining the thermal history of an environment whichmethod comprises:

locating within an environment, the thermal history of which it isdesired to determine, which environment cyclically varies above andbelow a predetermined threshold temperature, a sensing elementfabricated from a material which below a predetermined thresholdtemperature is stable and which at or above said threshold temperatureundergoes an irreversible predictable cummulative transformation in ameasurable physical property thereof as a function of the time theelement is exposed to a temperature equal to or greater than thethreshold temperature which sensing element is prepared by fabricating asensing element from an alloy having the characteristic of undergoing atransformation resulting in a change in physical properties, includingresistivity thereof, when the alloy is heated to said preselectedthreshold temperature; heat treating said sensing element at saidthreshold temperature for a predetermined time sufficient to establish apreselected resistance of the sensing element; and quenching saidsensing element upon expiration of said predetermined time of heattreating; and

measuring, after a selected interval of time, a change in resistivity ofthe sensing element thereby determining the thermal history of theenvironment.

12. The method of claim 11 which includes a plurality of sensingelements, each element having a predetermined different thresholdtemperature and wherein each element is measured separately to determinethe time which each element has been exposed to a temperature equal toor above the threshold temperature of that particular element therebydetermining the thermal history of the environment.

13. The method of claim 11 wherein the alloy is a nickel alloy and thethreshold temperature is above 1300" F.

14. The method of claim 11 wherein the alloy is agehardened for overhours.

References Cited by the Examiner V UNITED STATES PATENTS 1,643,582 9/27Martin 338-28 2,271,975 2/42 Hall 338-30 2,543,177 2/51 Korsgren 73--3622,714,054 7/55 Voltz 338-40 2,782,137 2/57 Jackson 148158 2,932,971 4/60Moore et al 73193 X OTHER REFERENCES Process and Physical Metallurgy, byJames E. G arside, published by Charles Griffen and Co. Ltd., 1949,(pages 211-214 relied on).

ISAAC LISANN, Primary Examiner.

1. A THERMAL HISTORY GAGE FOR PROVIDING AN INDICATION OF THE LENGTH OFTIME AN ENVIROMENT HAS BEEN AT OR ABOVE EACH OF A PLURALITY OFTEMPERATURE, COMPRISING A FRAME, AND A PLURALITY OF ELECTRICALLYCONDUCTIVE SENSING ELEMENTS CARRIED BY THE FRAME, EACH OF SAID SENSINGELEMENTS BEING FABRICATED FROM A MATERIAL WHICH BELOW A THRESHOLDTEMPERATURE IS STABLE AND WHICH AT OR ABOVE A THRESHOLD TEM-